Both the alternative-energy and flat-panel display markets have a need for high-quality, flexible substrates on which to produce highly crystalline semiconductor thin films.
The current solar cell (i.e., photovoltaic) market relies on technology that has been essentially unchanged for decades. Most of the market is served by crystalline Si, either single-crystal or polycrystalline, with average conversion efficiencies of 12-20%. The costs of crystalline Si devices are high due to high-cost production methods and high demand for the raw materials in competition with the semiconductor electronics industry. Si devices also require thick silicon structures to achieve these efficiencies, consuming significant quantities of material. The remainder of the market is served largely by thin-film structures based on amorphous Si that is cheaper to produce but has typical energy conversion efficiencies below 7%. Amorphous Si efficiencies also degrade with time.
Higher conversion efficiencies, over 30%, have been demonstrated for thin film multi junction devices based on III-V semiconductors such as GaAs. However, their production costs are very high since these devices are most advantageously grown on expensive single-crystal Ge or GaAs wafers.
Emerging low-cost photovoltaic technologies include ribbon-grown Si, copper-indium-gallium-selenide and cadmium telluride thin films, polymeric/organic films, and nanotechnology-based approaches. None of these approaches fully realizes the objectives of increased production volume, increased efficiency and lower cost per watt generated.
What is needed is a method for the low-cost production of large areas of high-efficiency photovoltaic conversion cells.
A useful substrate for the growth of high-efficiency semiconductor films (e.g., III-V semiconductor films) preferably enables the growth of low-defect films (similar to those formed on single-crystal wafers) but at lower cost and over larger areas. Flexibility is also a useful attribute. The substrate should also be chemically compatible with both the semiconductor material and with the semiconductor process environment. The substrate coefficient of thermal expansion and lattice constant preferably match the semiconductor as closely as possible. These demanding attributes restrict the number of materials that may effectively be used.
Photovoltaic cells produced from polycrystalline Si wafers constitute a significant proportion of the current solar-power market, and III-V cells with useful performance have been produced from polycrystalline Ge wafers. However, especially in the case of Ge, the cost of the polycrystalline wafer constitutes a considerable barrier to broader adoption of solar power for consumer use. Polycrystalline wafers are also fragile and heavy, limiting their application in building-integrated designs.
Prior attempts to produce polycrystalline III-V directly on low-cost ceramic or metal foil substrates have been unsuccessful. III-V compounds such as GaAs require very high recrystallization temperatures and the vast difference in vapor pressures between the III and V elements may result in depletion of one element. As noted above, however, due to their fairly close lattice matching to Ge, various III-V compounds for photovoltaics may be advantageously deposited on Ge substrates. This technique is principally limited by its expense, but the resulting photovoltaic cells are also typically heavy and brittle, making them unsuitable for many applications.
The ability to utilize cheaper, flexible polycrystalline Ge-based substrates for photovoltaics would address many of the above challenges, but grain boundaries generally have a deleterious impact on photovoltaic cell performance. Thus, techniques are needed to advantageously select and/or enlarge the grain size of polycrystalline semiconductor films while at the same time minimizing any deleterious impact of processing (e.g., at elevated temperatures) upon such films.